Your computer you are trying to download McAfee on may already has a virus on it and the message you are receiving from it is a fake alert as virus have a tendency to protect themselves.
McAfee has a link dealing with this Required Reading - Home User Assistance Malware Troubleshooting
You could also try running www.malwarebytes.org and www.superantispyware.com both programs are free and catches a lot of stuff. You may need to rename the download file and also the installation folder to something you can easily remember. You might also need to do this on a usb stick from a non infected computer. After you get them installed check for the latest updates and run the scan. Let it clean everything it finds and reboot the computer. Then do the same for the other program.
Let us know this works for you.